Electrical control apparatus



July 11, 1939. WA. TOLSON 2,165,770

ELECTRICAL CONTROL APPARATUS Filed April 28, 1934 3 Sheets-Sheet 1 I.film"- a w, w p: '93 u, 9 r WI I HII 7.3

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Patented July 11,1939

PATENT OFFICE 2,165,770 ELECTRICAL coN'raoL APPARATUS William A. Tolson,Westmont, N. J., assignor to Radio Corporation of America, a corporationof Delaware Application April 28,

8 Claims.

My invention relates tovelectrical control apparatus and moreparticularly to methods of and means for maintaining synchronousoperation in television systems and the like.

In television systems utilizing a cathode-ray receiver tube, it is thegeneral practice to synchronize the transmitter and receiver bytransmitting a horizontal synchronizing impulse at the end of eachscanning line and a vertical synchronizlng or framing impulse at the endof each picture frame. At the receiver, the horizontal synchronizingimpulses and the framing impulses are separated from the picture signalsand from each other and impressed upon saw-tooth wave generators whichsupply the deflecting current or voltage for the cathode-ray receivertube. In such a system, the saw-tooth wave generators oscillate underthe control of the synchronizing impulses.

In order to facilitate the separation of the horizontal synchronizingimpulses from the vertical synchronizing or framing impulses, it hasbeen found desirable to give them different wave shapes whereby theframing impulses contain considerably more energy than the horizontalsynchronizing impulses. Such a system is disclosed in application SerialNo. 565,953, filed September 30, 1931, in the name of R. D. Kell, andassigned to the same assignee as this application.

Experience has shown that vertical framing impulses contain sufiicientenergy to make the action of the vertical saw-tooth wave generatordependent upon the amplitude and shape of a framing impulse. This isespecially objectionable in television systems employing interlacedscanning since the scanning lines of one picture frame should fallbetween the lines of a preceding frame with all the lines equally spacedapart.

The difference in the amplitude and/or shape of the successive framingimpulses may be due to various causes, but in an interlaced scanningsystem the main reason for such diiference is the fact that alternateframing impulses have the adjacent horizontal synchronizing impulse adifierent time interval away in order to produce the interlaced scanningefiect. As the apparatus for transmitting a picture having interlacedscanning is no part of my invention, no description of it will be given.Unless special precautions are taken, the result of alternate framingimpulseshaving different wave shapes is that the interlaced scanninglines at the receiver tend to pair, Accordingly, either the lines areprominent in the picture or picture detail is lost.

It is, therefore, an object of my invention to 1934, Serial No. 722,843

provide an improved method of and means for maintaining synchronism intelevision systems.

It is a further object of my invention to provide an improved receiverfor use in a television system of the interlaced scanning type.

It is a further object of my invention to provide a system in which theoperation of an oscillator controlled by a plurality of periodicimpulses is substantially independent of the amplitude and/or wave shapeof said impulses.

A still further object of my invention is to provide a televisionreceiver in which positive synchronizing action is obtained without theuse of manually operated adjusting devices.

A still further object of my invention is to provide a televisionreceiver in which the saw-tooth wave oscillators need not be constructedwithin close limits for satisfactory synchronization.

A still further object of my invention is to provide improved means forreshaping an electrical impulse.

In practicing my invention, instead of impressing the framing impulsesdirectly upon the oscillator of the vertical deflecting circuit at thereceiver, I interpose between the said oscillator and the synchronizinginput what will be referred to as a driver tube. The driver tube is soconnected with .an associated circuit that it will operate to produce asharp voltage impulse in response to the application of a suitablevoltage impulse to its control grid. In a preferred embodiment of myinvention, the driver tube is so connected that it will start tooscillate in .response to, and only in response to, the reception of asynchronizing impulse. Also, the adjustment is such that after eachsynchronizing impulse, the driver tube assumes its normal inactivestate. Thus, each framing impulse causes a sharp voltage impulse toappear in the output circuit of the driver tube for controlling theoscillator of the deflecting circuit, this voltage impulse containingmuch less energy than the framing impulse and so little energy thatchanges in its amplitude and/or wave shape will have substantially noeffect upon the oscillator.

Other objects, features and advantages of my invention will appear fromthe following description taken in connection with the accompanyingdrawings in which:

Fig, 1 is a circuit and a block diagram of a television receiverembodying my invention,

Fig. 2 is a set of curves which are referred to in explaining theoperation of my invention, and

Figs. 3 and 4 are circuit diagrams of other embodiments.

Referring to Fig. 1, the invention is shown applied to a televisionreceiver in which a radio receiver and an amplifier 3 receive andamplify incoming television signals and impress them upon thecontrol-grid 1 of a cathode-ray receiver tube 5. Both picture signalsand synchronizing impulses are impressed upon this control grid.

The synchronizing impulses are separated from the picture signals bymeans of a suitable separating circuit indicated at 9 before beingapplied to the deflecting circuits. This separating circuit alsoseparates the horizontal synchronizing impulses from the framingimpulses, the horizontal synchronizing impulses being supplied to asaw-tooth wave generator indicated at H for supplying saw-tooth currentto the horizontal deflecting coils |3 of the cathode-ray tube 5. A

separating circuit of this type is disclosed in the above-mentioned Kellapplication.

The framing impulses are supplied through a conductor I5 and apotentiometer H to the vertical deflecting circuit. The saw-tooth wavegenerator for supplying saw-tooth current to the vertical deflectingcoils IQ of the cathode-ray tube 5 comprises an oscillator 2|, animpulse tube 23 and an output tube 25. The oscillator 2| is illustratedas a blocking oscillator although it should be understood that in placeof a blocking oscillator, other types of oscillators, such asoscillators of the Dyna-tron or multivibrator types, may be employed.The oscillator 2| comprises an electric discharge tube 21 of thesuppressor grid type which has a cathode 23, a control grid 3|, a screengrid 33, a suppressor grid 35, and an anode 31. The control grid 3| isconnected through a fixed grid leak resistor 39 and a variable grid leakresistor 4| to ground, and through ground to the cathode 23.

The grid circuit of the oscillator also includes a grid condenser 43,the secondary winding 45 of the transformer 41, and a section of apotentiometer 49, connected in series between the control grid 3| andground. The anode 31 is connected through the primary 5| of. thetransformer 41 to a suitable source of positive potential (not shown)for feeding energy from the plate circuit into the grid circuit. Thecoupling thus provided between the plate and grid circuits is such that,as the plate current through the primary winding 5| increases, thecontrol grid 3| is made more positive. The resulting positive potentialon the control grid 3| causes a flow of grid current which charges thegrid condenser 43 in a direction such that it tends to make the controlgrid negative. The plate current reaches a maximum value and thendecreases, at the same time reversing the direction of the inducedvoltage in the grid circuit whereby the control grid is made so negativethat the tube is biased beyond cut-off.

The grid condenser 43 has been charged suflicienily to bias the controlgrid 3| beyond the cutofi' point in the absence of any other voltage inthe grid circuit. Therefore, no current will flow again in the platecircuit until the charge has leaked off the condenser 43 through thegrid leak resistors 33 and 4| to lower the negative bias on the controlgrid a sufiicient amount. By properly adjusting the values of thevarious elements in the oscillator circuit, and, in particular, byproperly adjusting the values of the grid condenser and the grid leakresistors, the oscillator may be made to oscillate at the desiredpicture or frame frequency. g

In practice, the oscillator is adjusted to as in cillate at a frequencysomewhat lower than the desired frequency in order to permitsynchronization. For example, if 60 picture frames per second aredesired, the oscillator may be so adjusted that it tends to oscillate at57 cycles per second.

In accordance with my invention, the synchronizing signals are impressedupon the oscillator through a driver tube 53 which may be of thesuppressor grid type including a cathode 55, a control grid 51, a screengrid 59, a suppressor grid BI, and an anode 63. As will appear from thefollowing description, the driver tube 53 is so connected that it wouldfunction as a blocking oscillator except for the fact that it isnormally biased beyond cut-off.

The control grid 51 is connected through a grid-leak resistor and abiasing battery 61 to ground and through ground tothe cathode 55. Thegrid circuit also includes a grid condenser 69 which is connected inseries with a winding 1| of a transformer 13 and a section of thepotentiometer l1.

The plate circuit of the driver tube 53 is coupled to its grid circuitthrough a winding 15 of the transformer 13, the anode 63 being suppliedwith positive potential through this transformer winding. The couplingbetween the windings 1| and 15 is such that, except for the biasingbattery 61, the tube would oscillate as a blocking oscillator the sameas oscillator 2|. The potential applied to the control grid 51 by thebiasing battery 61 is suiiicient, however, to bias the driver tubebeyond the cut-off point whereby it is inactive until a synchronizingimpulse is impressed thereon.

The transformer is provided with a third winding 11 which has thepotentiometer 49 connected thereacross for impressing a portion of thevoltage output of the driver tube upon the blocking oscillator circuit.In order to prevent an undesirable amount of reaction from the blockingoscillator 2| upon the driver tube 53. the blocking oscillator isconnected across only a small percentage of the potentiometer 49 asindicated by the potentiometer values which are given in the drawings byway of example.

The above-described circuit operates to produce voltage impulses in theblocking oscillator circuit 2| which are under the control of thesynchronizing impulses. As previously mentioned, this control isobtained by adjusting the oscillator 2| to operate at a frequencyslightly lower than that of the synchronizing impulses whereby asynchronizing impulse (reshaped by driver tube 53) increases the voltageon the control grid 3| above the cut-off point at an instant just beforethis voltage change would be made by the oscillator itself.

The voltage impulses generated by the blocking oscillator 2| areimpressed upon the tube 23 which is provided for the purpose of soaltering the shape of the voltage impulse that a perfect saw-toothcurrent will be supplied to the deflecting coils l3. This change in waveshape is obtained by means of a condenser 19 and a reslstor 3| connectedin series in the output circuit of the tube 23.

A voltage wave, having a saw-tooth shape. appears across the condenser19. while a voltage impulse wave appears across the resistor 8|. the twowaves combining to give the proper wave shape for causing a puresaw-tooth wave of current to flow through an inductance coil containg acertain amount of resistance.

The deflecting coils I9 are connected to the output circuit of theoutput tube 25 through a direct current connection including thevariable tap 83 and resistors 85 for properly centering the cathode-raybeam on the fluorescent screen of the tube 5, and through an alternatingcurrent or impulse connection which includes a bypass condenser 81. I

The manner in which a synchronizing impulse containing a large amount ofenergy is converted into a synchronizing impulse containing a relativelysmall amount of energy will be better understood by referring to thecurves in Fig. 2 which indicate wave shape and phase relation but notrelative magnitude. In this figure, the voltage across the potentiometerI1 of the framing impulse circuit is represented by the curve 89, whilethe voltage appearing in the grid circuit of the driver tube 53 isrepresented by the curve 9|.

Referring to the curves 89 and 9|, it will be seen that at the time hthe voltage on the control grid 51 of the driver tube 53 is thatsupplied by the fixed bias battery 81 and is below the cut-off point ofthe driver tube. At the time t2 the grid potential of the driver tube 53has been raised by the incoming synchronizing impulse to the cut-offpoint, at which point plate current begins to fiow and the cycle ofoperation is begun as shown. This cycle of operation is a typicalblocking oscillator cycle except that at the time is when the charge hasleaked off the grid condenser 69, the cycle does not repeat but,instead, the circuit is maintained inoperative because of the fixed biason the control grid 51. The time from t2 to is may be referred to as theactive period of the oscillator. The cycle of operation is notrepeatedagain until the next synchronizing impulse occurs.

It will be apparent by comparing the area under a framing impulse shownin curve 89 with the area under that portion of the driver tube voltageshown in curve 9| above the zero axis that there is much less energy inthe synchronizing output of the driver tube 53 than in the framingimpulses. For this reason, slight changes in the output of the drivertube will have practically no effect upon the blocking oscillator 2|. Inparticular, it has beeniound that by employing a driver tube, asdescribed, there is no appreciable tendency for the scanning lines topair due to changes in the relative spacing of horizontal synchronizingimpulses and framing impulses in interlaced scanning systems.

Attention is called to the fact that the driver tube 53 is so adjustedthat, with the fixed grid bias removed, it will oscillate to producepositive votage peaks which have a smaller time interval between themthan do the vertical synchronizing impulses. That is, the intervalbetween t2 and ta (Fig. 2) is less than the interval between framingimpulses. The reason for this adjustment is that it permits all thecharge to leak ofi the grid condenser 69 and so return the driver tubeto its original state before the next framing impulse occurs. Thisprevents the action of the driver tube during the reception of oneframing impulse from being influenced by a preceding framing impulse.

Also, it will be seen that the time interval between i2 and ta should begreat enough to prevent a single framing impulse from causing the drivertube to break into oscillation more than once In general, this meansthat the time interval between is and t3 should be greater than the timefrom the beginning of a framing impulse to its end. I

By referring to Figure 1,,it will be seen that there-ls only a smallamount of couplingbetween the driver tube 53 and the blocking oscillator2|, this coupling being obtained through the 200 ohm section of the10,200 ohm potentiometer 49. The coupling is made small since theblocking oscillator reacts on the driver tube and a large amount ofcoupling between the two tubes would result in oscillation of the drivertube circuit under the control of the blocking oscillator. With smallcoupling between the driver tube circuit and the oscillator circuit, theoperation of the deflecting circuit is perfectly satisfactory when theblocking oscillator is properly adjusted. In practice, this adjustment,which is referred to as a speed control adjustment, is made by varyingthe grid leak resistor 4| until the oscillator has a normal oscillatingperiod close to the desired picture or frame frequency. If theadjustment of the oscillator 2| is such that, when free from the controlof the driver tube 53, it will oscillate at a frequency widelydifferentfrom the frame frequency, the output of the driver tubewill'not pull the oscillator into synchronism.

In some television receivers, it may be desirable to eliminate a speedcontrol or frequency adjustment of the vertical deflecting circuit inorder to simplify the operation of the receiver. This may beaccomplished by employing a circuit, such as illustrated in Fig. 3, inwhich an amplifier tube 93 is employed for preventing reaction of theoscillator on the driver tube. While the circuit shown in Fig. 3 and thevertical deflecting circuit shown in Fig. 1 may be identical except forthe addition of the coupling tube 93, several variations have beenillustrated. In the two figures, like parts have been indicated by thesame reference numerals.

Referring to Fig. 3, the driver tube may comprise an electric dischargetube 95 having a cathode 91, a control grid 99 and an anode IOI. Thecontrol grid 99 is connected through a gridleak resistor I03 and abiasing battery I05 to ground and through ground to the cathode 91, thebiasing battery I05 applying a negative bias to the grid 99 for biasingthe tube beyond cutofi. A grid-leak condenser I01, the secondary I09 ofa transformer III and the lower section of the synchronizing inputpotentiometer I1 areconnected in series between the control grid 99 andground.

The plate circuit of the tube 95 is coupled to the grid circuit by meansof the primary II3 of the transformer III. As in Fig. 1, this couplingand the constants of the driver tube circuit' are so adjusted that thedriver tube would oscillate to supply voltage impulses if it were notfor the biasing battery I05.

The blocking oscillator comprises an electric discharge tube II5 havinga cathode H1, a control grid H9 and an anode I2I. The control grid H9 isconnected through a winding I23 of a transformer I25 and a grid-leakresistor I21 to ground, and through ground to the cathode III. Thegrid-leak resistor I21 is shunted by a grid condenser I29 which chargesup to apply a negative potential to the control grid II9 during aportion of the operating cycle as explained in connection with theblocking oscillator shown in Fig. 1. The plate circuit of the tube iscoupled in the proper phase relation to the grid circuit by means of awinding I3I of the transformer I25.

The voltage impulses appearing in the output circuit of the driver tube95, are impressed upon the blocking oscillator circuit through theelectric discharge tube 93, which has a cathode I33, a control grid Iand an anode I31. The anode Ill of the driver tube 95 is coupled to thecontrol grid I35 of the coupling tube 93 through a coupling condenserI39. The control grid I35 is maintained at a negative potentialsufficient to bias tube 93 beyond cut-off by means of a biasing batteryI which has its positive terminal connected to the cathode I33 throughground and its negative terminal connected to the grid I35 through agrid leak resistor I43. The output circuit of the coupling tube 93 iscoupled to the blocking oscillator circuit through a third winding I45on the transformer I25. It will be understood that the winding I45 iswound or connected in such a direction that a synchronizing impulse inthe output of the tube 93 makes the grid II9 of the blocking oscillatorII5 less negative.

While it is preferred to bias the tube 93 beyond cut-off whereby onlythe positive voltage peaks of the driver tube output are impressed uponthe blocking oscilator II5, the tube 93 may be biased to function as anordinary class A amplifier. By biasing the tube 93 beyond cut-off,however, extraneous voltages which might interfere with propersynchronization of scanning are eliminated.

The blocking oscillator H5 is coupled to the impulse tube 23 forsupplying a voltage wave to the input circuit of the output tube 25 ofthe proper wave form for producing a saw-tooth current in the verticaldeflecting coils. The blocking oscillator .I I5 may be coupled to theimpulse -tube 23 in the way shown in Fig. 1, or it may be coupledthereto by means of a direct conection from the grid II9 of the blockingoscillator tube to the grid I41 of the impulse tube 23 through aconductor I49, as shown in Fig. 3. The main distinction between thesetwo coupling connections is that in Fig. 3 the control grid of theimpulse tube 23 derives a suitable biasing potential from the gridcircuit of the oscillator tube II5 instead of from its own grid-leakresistor as in Fig. 1. As in Fig. 1, the bias on the impulse tube 23 issuflicient to bias it beyond cut-off. It will be noted that because ofthe voltage drop in the plate resistor I46, a grid voltage insufficientto bias tube Hi to cut-off does bias tube 23 to cutoff.

I have found that by utilizing the coupling tube 93 between the drivertube and the deflecting oscillator, it is not necessary to make anyclose adjustment of the oscillator frequency in order to insure propersynchronization. For example, assuming that the synchronizing impulsesare occurring at the rate of 60 per second, in order to produce 60picture frames per second, the oscillator may be adjusted to oscillateat a frequency as low as cycles per second and it will be pulled intothe synchronism by the synchronizing impulses. Not only does this permitthe elimination of a speed control" adjustment, but it also insures thata receiver will synchronize properly even through some slight change orvariation has been made in the oscillator circuit. Such changes orvariations may result from replacing a defective oscillator tube or theymay occur during the manufacture of the receivers as an unavoidableresult of quantity production methods.

As previously mentioned, oscillator circuits of other types may beutilized in place of a blocking oscillator circuit in practising myinvention. In Fig. 4, there is illustrated a circuit of the Dynatrontype in which the tube is properly biased to function as a driver tubefor a saw tooth wave generator. In Figs. 1 and 4, like parts are indicated by the same reference numerals.

Referring to Fig. 4, the driver tube comprises an electric dischargetube I50 having a cathode I5I, a control grid I53, 2. screen grid I55,and an anode I51. The anode I5! is connected through an inductance coilI59 to a source of potential such as a battery I6I while the screen gridI55 is connectedto a point of higher potential on the battery I6I suchthat the tube would oscillate as a Dynatron relaxation oscillator if thegrid I53 were connected directly to the cathode I5I.

The grid I53, however, is negatively biased by a biasing source such asa battery I63, the bias being suflicient to bias the tube I50 tocut-oil. Therefore, the tube I50 will start to oscillate only inresponse to the appearance of a synchronizing impulse across theresistor I1 and will reach an inactive state before the occurrence ofthe next synchronizing impulse. As in' the case of the circuit of theblocking oscillator type, the tube I50 converts a framing impulse into asharp voltage impulse containing a small amount of energy.

From the foregoing description, it will be apparent that variousmodifications may be made in my invention without departing from thespirit and scope thereof, and I desire, therefore, that only suchlimitations shall be placed thereon as are necessitated by the prior artand set forth in the appended claims.

I claim as my invention:

1. In combination apparatus for converting a plurality of electricalimpulses having a certain characteristic and occurring at regularlyspaced time intervals into a plurality of electrical impulses having adifferent characteristic, said apparatus including an oscillator of theself-oscillatory type comprising an electric discharge tube of the highvacuum type having a plurality of electrodes, means for causing saidoscillator to oscillate and then become inactive in response to theapplication of a certain potential to one of said electrodes, the timeinterval between the start of said oscillation and the instant the tubebecomes inactive being less than said regularly spaced time intervalsand being greater than the duration of each of said first impulses,means for applying said certain potential to said one electrode inresponse to the occurrence of each of said first impulses, means forproducing recurring electrical waves of saw-tooth shape in response torecurring electrical waves being impressed thereon, and means forimpressing the impulses produced by said apparatus upon said last means.

2. In combination, apparatus for reshaping an electrical impulse, saidapparatus comprising an electric discharge tube having a cathode, acontrol grid, and a plate, a condenser and a grid leak resistorconnected between said control grid and said cathode to form a gridcircuit, said tube having an output circuit including said plate, meansfor so coupling said output circuit to said grid circuit that said gridis made less negative in response to an increase in plate current, meansfor so biasing saidgrld that said tube is maintained biased beyond thecut-off point until said impulse is impressedupon said grid circuit froman outside source, means for impressing said impulses upon said gridcircuit, and an oscillator which oscillates freely in an uncontrolledcondition, said apparatus and said oscillator being so.

coupled that said reshaped impulses control said oscillator.

3. The method of operating an oscillator 01' the self-oscillatory typecomprising an electric discharge tube of the high vacuum typehaving acontrol electrode and so connected as to generate voltage impulseshaving a certain period, which method consists in applying a constantbiasing potential to said control electrode such that the tube is biasedbeyond, cut-off until control im- 4. In a television system of the typein which horizontal synchronizing impulses occurring at a certainfrequency and having a certain energy content and vertical synchronizingimpulses occurring at a lower frequency and having a greater energycontent are transmitted for synchronizing the scanning at thetransmitter with that at the receiver, the method of synchronizing whichcomprises separating said horizontal synchronizing impulses from saidvertical synchronlzing impulses at the receiver by a filtering actionwhereby said vertical impulses only may,

be impressed upon a self-oscillatory vertical defleeting circuit at thereceiver, next converting said vertical synchronizing impulses intoimpulses of shorter duration and'having a smaller energy content, andimpressing said converted impulses upon the self-oscillatory verticaldefleeting circuit for synchronizing the scanning at the receiver withthat at the transmitter.

5. In combination, at least two cascade-connected oscillators, the firstoscillator having a natural period higher than a predetermined frequencyand the second oscillator having a natural period lower than the saidpredetermined frequency, means for normally biasing the first oscillatortorender it inactive and means for periodically supplying controlimpulses at the predetermined frequency tothe first oscillator tocounteract the bias, whereby the second oscillator is constrained tooperate in synchronism with the said impulses.

6. In a television system, a saw-tooth wave generator which includes aself-oscillatory oscillator, means for converting synchronizing impulsescontaining a certain amount of energy into impulses having an amplitudeat least as great as the amplitude of the first impulses but containingless ener y, said means including an electric discharge tube of the highvacuumptypefhaving a plurality of electrodes and including means for,causing said tube to oscillate momentarily and then become inactive inresponse to the appliflcation to one of said electrodes of one of saidsynchronizing impulses, and means-for impressing said convertedimpulsesupon said. oscillator whereby said saw-tooth waves are producedin synchronism with said synchronizing impulses.

7. In a television system of the type in which certain frequency andhaving a certain energy content and vertical synchronizing impulsesoccurring at a lower frequency and having a greater energy content aretransmitted for synchronizing the scanning at the transmitter with thatat the receiver, said receiver being of the type including a verticaldeflecting circuit having a self-oscillatory circuit, the method ofsynchronizing which comprises separating said horizontal synchronizingimpulses from said vertical synchronizing impulses at the receiver by afiltering action, next converting said vertical synchronizing impulseinto impulses of shorter duration and having a smaller energy content,and impressing said converted impulses upon said self-oscillatorycircuit for synchronizing the scanning at the receiver with that at thetransmitter.

8. Apparatus for converting periodically recurring electrical impulseseach having a certain amplitude and energy content into periodicallyrecurring electrical impulses each having an amplitude at least as greatas said certain amplitude and an energy content less than said certainenergy content, said apparatus comprising a high vacuum electricdischarge tube having a plate, a cathode and a control grid, atransformer having a primary and a secondary, a grid circuit connectedbetween said grid and said cathode and including said secondary, a platecircuit conductive to direct current connected between said plate andsaid cathode and including said primary, said plate and grid circuitsbeing coupled through said transformer in such phase as to produceoscillations, a condenser in said grid circult in series with saidsecondary for storing a charge to block said tube, a grid resistor and abiasing voltage source in series with each other and in parallel to saidcondenser whereby a charge may leak off said condenser, said biasingsource being connected in the correct polarity and the biasing voltagebeing of sufficient magnitude to block said tube in the absence of acharge on said condenser, the capacity of said condenser and theresistance of said grid resistor having such relative values that in theabsence of said biasing voltage said tube produces voltage impulseswhich recur at time intervals longer than said first-mentioned impulses,and circuit connections for impressing said first-mentioned impulsesupon said control grid in a direction to make it less negative.

WILLIAM A. TOLSON.

. l0 horizontal synchronizing impulses occurring at a I

